THz Transistors, sub-mm-wave ICs, mm-wave Systems , fax 20th Annual Workshop on Interconnections within High Speed Digital Systems, Santa Fe, New Mexico, 3 – 6 May 2009 Mark Rodwell University of California, Santa Barbara

The End (of Moore's Law) is Near (?) It's a great time to be working on electronics ! Things to work on: InP transistors: extend to 3-4 THz→ GHz & low-THz ICs GaN HEMTs: powerful transmitters from GHz Si MOSFETs: scale them past 16 nm VLSI transistors: subvert Boltzmann→ solve power crisis III-V MOSFETs: help keep VLSI scaling (maybe) mm-wave VLSI: massively complex ICs to re-invent radio Our focus today: THz transistors - how and why

Why THz Transistors ?

Why Build THz Transistors ? THz amplifiers→ THz radios → imaging, sensing, communications precision analog design at microwave frequencies → high-performance receivers 500 GHz digital logic → fiber optics Higher-Resolution Microwave ADCs, DACs, DDSs

What Would You Do With a THz Transistor ? 640 Gb/s ETDM optical fiber links GHz imaging systems mm-wave communication links

At High Frequencies The Atmosphere Is Opaque Mark Rosker IEEE IMS 2007

What Else Would You Do With a THz Transistor ? precision, high-performance analog microwave circuits higher-resolution microwave ADCs, DACs, DDSs

How to Make THz Transistors

Simple Device Physics: Resistance Good approximation for contact widths less than 2 transfer lengths. bulk resistancecontact resistance -perpendicular contact resistance - parallel

Simple Device Physics: Depletion Layers capacitancetransit timespace-charge limited current

Simple Device Physics: Thermal Resistance Exact Carslaw & Jaeger 1959 Long, Narrow Stripe HBT Emitter, FET Gate Square ( L by L ) IC on heat sink

Simple Device Physics: Fringing Capacitance wiring capacitanceFET parasitic capacitances VLSI power-delay limits FET scaling constraints

Frequency Limits and Scaling Laws of (most) Electron Devices To double bandwidth, reduce thicknesses 2:1Improve contacts 4:1 reduce width 4:1, keep constant length increase current density 4:1 PIN photodiode

parameterchange collector depletion layer thicknessdecrease 2:1 base thicknessdecrease 1.414:1 emitter junction widthdecrease 4:1 collector junction widthdecrease 4:1 emitter contact resistancedecrease 4:1 current densityincrease 4:1 base contact resistivitydecrease 4:1 Changes required to double transistor bandwidth: Linewidths scale as the inverse square of bandwidth because thermal constraints dominate. Bipolar Transistor Scaling Laws

parameterchange gate lengthdecrease 2:1 gate dielectric capacitance densityincrease 2:1 gate dielectric equivalent thicknessdecrease 2:1 channel electron densityincrease 2:1 source & drain contact resistancedecrease 4:1 current density (mA/  m) increase 2:1 Changes required to double transistor bandwidth: Linewidths scale as the inverse of bandwidth because fringing capacitance does not scale. FET Scaling Laws

Thermal Resistance Scaling : Transistor, Substrate, Package

Probable best solution: Thermal Vias ~500 nm below InP subcollector...over full active IC area.

Electron Plasma Resonance: Not a Dominant Limit

THz & nm Transistors: it's all about the interfaces Metal-semiconductor interfaces (Ohmic contacts): very low resistivity Dielectric-semiconductor interfaces (Gate dielectrics): very high capacitance density Transistor & IC thermal resistivity.

THz Bipolar Transistors

InP Bipolar Transistor Scaling Roadmap emitter nm width  m 2 access  base nm contact width,  m 2 contact  collector nm thick, mA/  m 2 current density V, breakdown f  GHz f max GHz power amplifiers GHz digital 2:1 divider GHz industryuniversity →industry university appears feasible maybe

InP DHBTs: September 2008

512 nm InP DHBT Production DDS IC: 4500 HBTs20-40 GHz op-amps 500 nm mesa HBT 150 GHz M/S latches175 GHz amplifiers Laboratory Technology Teledyne / BAETeledyne / UCSB UCSBUCSB / Teledyne / GCS 500 nm sidewall HBT f  = 405 GHz f max = 392 GHz V br, ceo = 4 V Teledyne 20 GHz clock dBm 2 GHz with 1 W dissipation ( Teledyne ) Z. Griffith M. Urteaga P. Rowell D. Pierson B. Brar V. Paidi

256 nm Generation InP DHBT 150 nm thick collector 70 nm thick collector 60 nm thick collector 200 GHz master-slave latch design Z. Griffith, E. Lind J. Hacker, M. Jones 324 GHz Amplifier

324 GHz Medium Power Amplifiers in 256 nm HBT ICs designed by Jon Hacker / Teledyne Teledyne 256 nm process flow- Hacker et al, 2008 IEEE MTT-S ~2 mW saturated output power

InP Bipolar Transistor Scaling Roadmap emitter nm width  m 2 access  base nm contact width,  m 2 contact  collector nm thick, mA/  m 2 current density V, breakdown f  GHz f max GHz power amplifiers GHz digital 2:1 divider GHz industryuniversity →industry university appears feasible maybe

Conventional ex-situ contacts are a mess textbook contact with surface oxide Interface barrier → resistance Further intermixing during high-current operation → degradation with metal penetration THz transistor bandwidths: very low-resistivity contacts are required

Ohmic Contacts Good Enough for 3 THz Transistors 64 nm (2.0 THz) HBT needs ~ 2  - μm 2 contact resistivities 32 nm (2.8 THz) HBT needs ~ 1  - μm 2 Contacts to N-InGaAs*: Mo MBE in-situ 2.2 (+/- 0.5)  - μm 2 TiW ex-situ ~2.2  - μm 2 Contacts to P-InGaAs: Mo MBE in-situ below 2.5  - μm 2 Pd/...ex-situ ~1 (+/- ?)  - μm 2 *measured emitter resistance remains higher than that of contacts.

Process Must Change Greatly for 128 / 64 / 32 nm Nodes Undercutting of emitter ends control undercut → thinner emitter thinner emitter → thinner base metal thinner base metal → excess base metal resistance {101}A planes: fast {111}A planes: slow

128 / 64 nm process: where we are going Developing scalable self-aligned base process 2  -  m 2 resistivity base contacts - in-situ, in MBE 0.3  -  m 2 resistivity emitter contacts - in-situ, in MBE target ~2000 GHz device

THz Field-Effect Transistors (THz HEMTs)

parameterchange gate lengthdecrease 2:1 gate dielectric capacitance densityincrease 2:1 gate dielectric equivalent thicknessdecrease 2:1 channel electron densityincrease 2:1 source & drain contact resistancedecrease 4:1 current density (mA/  m) increase 2:1 Changes required to double transistor bandwidth: InGaAs HEMTs are best for mm-wave low-noise receivers......but there are difficulties in improving them further. FET Scaling Laws

Why HEMTs are Hard to Improve III-V MOSFETs do not face these scaling challenges SourceDrain Gate K Shinohara 1 st challenge with HEMTs: reducing access resistance gate barrier lies under S/D contacts → resistance low electron density under gate recess→ limits current 2 nd challenge with HEMTs: low gate barrier high tunneling currents with thin barrier high emission currents with high electron density channel gate barrier

III-V MOSFETs for VLSI→ Also Helps HEMT Development What is it ? MOSFET with an InGaAs channel What are the problems ? low electron effective mass→ constraints on scaling ! must grow high-K on InGaAs, must grow InGaAs on Si Why do it ? low electron effective mass→ higher electron velocity more current, less charge at a given insulator thickness & gate length very low access resistance

III-V MOSFETs in Development Wistey Singisetti Burek Lee. Rodwell Gossard Mo S/D metal with N+ InAs underneath side of gate top of gate InAs regrowth gate Mo S/D metal with N+ InAs underneath

III-V MOSFETs as a Scaling Path for THz HEMTs Why ? Much lower access resistance in S/D regions Higher gate barrier→ higher feasible electron density in channel Higher gate barrier→ gate dielectric can be made thinner Estimated Performance (?) 2 THz cutoff frequencies at 32 nm gate length

Applications of THz III-V Transistors

What Else Would You Do With a THz Transistor ? precision, high-performance analog microwave circuits higher-resolution microwave ADCs, DACs, DDSs

input power, dBm output power, dBm linear response 2-tone intermodulation increasing feedback mm-wave Op-Amps for Linear Microwave Amplification Reduce distortion with strong negative feedback R. Eden DARPA / UCSB / Teledyne FLARE: Griffith & Urteaga measured GHz bandwidth measured 54 dBm 2 GHz new designs in fabrication simulated 56 dBm 2 GHz 300 GHz / 4 V InP HBT

What Would You Do With a THz Transistor ? 640 Gb/s ETDM optical fiber links GHz imaging systems mm-wave communication links

670 GHz Transceiver Simulations in 128 nm InP HBT transmitter exciter receiver LNA: 9.5 dB Fmin at 670 GHz 3-layer thin-film THz interconnects thick-substrate--> high-Q TMIC thin -> high-density digital 670 GHz (128 nm HBT) PA: 9.1 dBm Pout at 670 GHz VCO: -50 dBc (1 100 Hz offset at 620 GHz (phase 1) Dynamic divider: novel design, simulates to 950 GHz Mixer: 10.4 dB noise figure 11.9 dB gain

What Would You Do With a THz Transistor ? 640 Gb/s ETDM optical fiber links GHz imaging systems mm-wave communication links

150 GHz carrier, 100 Gbs/s QPSK radio: 30 cm antennas, 10 dBm power, fair weather→ 1 km range 150 & 250 GHz Bands for 100 Gb/s Radio ? GHz, GHz: enough bandwidth for 100 Gb/s QPSK Wiltse, 1997 IEEE APS-Symposium, sea level 4 km 9 km 150 GHz band: Expect ~10-20 dB/km attenuation for rain 300 GHz band: expect ~20-30 db/km from 90% humidity

Interconnects within high-speed ICs

+V 0V kzkz Microstrip mode Substrate modes +V 0V CPW mode 0V CPW has parasitic modes, coupling from poor ground plane integrity kzkz Microstrip has high via inductance, has mode coupling unless substrate is thin. We prefer (credit to NTT) thin-film microstrip wiring, inverted is best for complex ICs -V0V+V 0V0V Slot mode ground straps suppress slot mode, but multiple ground breaks in complex ICs produce ground return inductance ground vias suppress microstrip mode, wafer thinning suppresses substrate modes M. Urteaga, Z. Griffith, S. Krishnan

mm-wave MIMO: → Wireless Links at 100's of Gb/s

mm-wave (60-80 GHz) MIMO → wireless at 40+ Gb/s rates ? 70 GHz, 1 km, 16 elements, 2 polarizations, 3.6 x 3.6 meter array, 2.5 GBaud QPSK → 160 Gb/s digital radio ?

mm-wave MIMO: 2-channel prototype, 60 GHz, 40 meters

mm-wave MIMO: 4-channel prototype, 60 GHz, Indoor 4 x 622 Mb/s 60 GHz carrier

VSLI for mm-wave & sub-mm-wave systems

Billions of 700-GHz Transistors→ Imaging & Arrays 65 nm CMOS: ~5 dB 200 GHz 22 nm will be much faster yet. What can you do with a few billion 700-GHz transistors ? Build Transmitter / Receiver Arrays 100's or 1000's of transmitters or receivers...on < 1 cm 2 IC area...operating at GHz. 150 GHz 8 dB amplifier IBM: Janagathan, Pekarik UCSB: Seo

3-stage 250GHz Amplifier Design Fcenter= 260GHz Gain= 9.9dB P DC = 15mW IBM: Pekarik, Jaganathan UCSB: M. Seo Designs in Fabrication 32 nm CMOS

millimeter-wave spectrum: new solutions needed mm-wave Bands → Lots of bandwidth short wavelength→ weak signal → short range highly directional antenna → strong signal→ long range narrow beam→ must be aimed → no good for mobile monolithic beam steering arrays→ strong signal, steerable 32 x 32 array → dB increased SNR→ vastly increased range → multi-Gigabit mobile communications

Billions of 700-GHz Transistors→ Imaging & Arrays Arrays for point-point radio links: Arrays for (sub)-mm-wave imaging : Arrays for Spatial-Division-Multiplexing Networks:

THz Transistors

Why Build THz Transistors ? THz amplifiers→ THz radios → imaging, sensing, communications precision analog design at microwave frequencies → high-performance receivers 500 GHz digital logic → fiber optics Higher-Resolution Microwave ADCs, DACs, DDSs

THz Integrated Circuits Device scaling (Moore's Law) is not yet over. Challenges in scaling: contacts, dielectrics, heat Multi-THz transistors: for systems at very high frequencies for better performance at moderate frequencies Vast #s of THz transistors complex systems new applications.... imaging, radio, and more Scaling→ multi-THz transistors.

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MIMO Link: a Subsampled Multi-Beam Phased Array array places nulls at interfering transmitters